Internal combustion engines on the basis of Otto (gasoline) engines are generally operated with fuel from hydrocarbons, from fossil fuels based on refined crude oil. Ethanol produced from renewable resources (plants) or another kind of alcohol is increasingly being added in various mixing ratios to the fuel. In the USA and Europe a blend of 70-85% ethanol and 15-30% gasoline is often distributed under the trade name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with blends up to E85. This is denoted as a “flex-fuel operation”. The operating parameters in the flex-fuel operation have to be adapted in each case to the existing fuel blend for an efficient operation with only a small discharge of toxic emissions; while at the same time a high engine output and a good starting performance are guaranteed. A stoichiometric fuel-air ratio is, for example, present at 14.7 mass parts of air per part of gasoline; however, when using ethanol, a proportion of air of 9 mass parts must be set. Furthermore, the ignition angle of the internal combustion engine has to be adapted to the mixing ratio for optimal combustion. Due to the different evaporation characteristics of ethanol and gasoline, different richening factors have to be specified as a function of the mixing ratio during start-up of the internal combustion engine. The diagnostic thresholds, which are to be used for the self-diagnosis of the internal combustion engine, the so-called on-board diagnosis, are likewise dependent on the composition of the fuel blend. Knowledge of the prevailing fuel mixing ratio is therefore of fundamental importance for the operation of the internal combustion engine.
A method for adaptively adjusting a fuel/air mixture to take into account fuel characteristics during the operation of an internal combustion engine is known from the German patent DE 41 17 440 C2. Said internal combustion engine is provided with a lambda controller, which emits a control factor RF, and also with an adaptation integrator, which emits an adaptation factor AF with variable adaptation speed, which in addition to the control factor FR manipulates the adjustment of the fuel/air mixture. Provision is thereby made for a test to be made to determine whether the lambda closed-loop control deviation amplitude exceeds a first threshold value; and if this is the case, the adaptation speed is set to an increased value up until a specified condition is met. Thereafter said adaptation speed is set back to a low value. The underlying method is known as a fuel adaptation.
The fuel adaptation allows for internal combustion engines, which can be operated with different fuel blends, to be operated trouble-free. Hence, the duration of injection must be lengthened by more than 40% when changing from a fuel consisting of pure gasoline to a fuel blend of 85% ethanol and 15% gasoline in order to maintain the same lambda values in the exhaust gas. This stems from the different air requirement for a stoichiometric combustion. According to the method described in the German patent DE 41 17 440 C2, a corresponding adaptation intervention is additionally performed. Because a very pronounced correction of the durations of injection and thereby of the adaptation intervention must be performed in the event of a change in fuel blends in comparison to the effects of wear or manufacturing, the adaptation speed resulting from a detected change in fuel blends is significantly increased in the proposed method.
Repeated fueling of small quantities of fuel or a skewing of the determination of the fill level by the motor vehicle not being parked on a level surface can, however, lead to a faulty adaptation of the ethanol value and consequently can also complicate a correct adaptation of the diagnostic thresholds for the on-board diagnosis. An insufficient mixing of the fuel components can also have a troublesome effect on the adaptation.
Additional methods are known from the patent literature. Said methods allow for the different characteristics of the fuel being used, among other things due to the different mixing ratios of fossil fuels and alcohols, to be inferred and for a corresponding adaptation of the operating parameters of the internal combustion engine.
A method for optimizing the work cycle of an internal combustion engine with an externally-supplied ignition is known from the German patent DE 29 52 073 A1. In this method, power train data of the internal combustion engine and in fact among other things the current relative angular position of the crankshaft (crank angle) are measured and are provided to an electronic control unit, wherein said data are accordingly allowed to affect the ignition timing and/or the quantity of fuel supplied. In so doing, provision is made for at least one of these control variables to be varied from work cycle to work cycle around an approximate nominal value for the ignition timing and/or the quantity of fuel supplied, for the current indicated pressure or a variable, which analogously changes with it, to be continuously measured in addition to the crank angle, for the average indicated pressure, respectively an analogous variable, to be calculated in each case from said continuous measurements and from the piston position, which is ascertained versus the crank angle, during every power stroke and for the sequentially arranged, calculated values of the average indicated pressure, which are automatically indexed and consecutive values, to be compared with each other. In the process, the variation of the control variable is aborted and the prevailing value of the control variable is retained for each operating state of the internal combustion engine as soon as the average indicated pressure achieves a maximum value. The method allows among other things for the compensation for deviations from the control parameters in the open-loop control of the internal combustion engine, which arise from the use of different fuels, as, for example, methanol, ethanol and gasoline.
A device for the acquisition of the fuel characteristics for an internal combustion engine is known from the German patent DE 38 33 123 A1, wherein the quantity of intake air and the air/fuel ratio in the exhaust gas are measured, a base quantity of injected fuel is calculated on the basis of the quantity of intake air and the quantity of the fuel to be injected is controlled in a closed-loop corresponding to the air/fuel ratio. The device is characterized by wherewithal, which acquires pressure, for the acquisition of the internal cylinder pressure, wherewithal, which acquires crank angle, for the acquisition of the crank angle of the internal combustion engine and by a monitoring mechanism, which receives signals from the wherewithal, which acquires pressure, and the wherewithal, which acquires crank angle. Said monitoring mechanism calculates an effective heating value Q of the fuel in an ignition cycle on the basis of the internal cylinder pressure P (θ) at a crank angle in the compression and expansion (combustion) strokes of an ignition cycle, on the basis of the crank angle θ and on the basis of the cylinder capacity V (θ); and said monitoring mechanism ascertains an effective combustion value K or a lower heating value Hu of the fuel. In so doing, the characteristics of the fuel are acquired with the aid of at least the effective combustion value K or the lower heating value Hu or the ratio (Ti/Hu) of the duration Ti of a fuel injection pulse to the heating value Hu.